Method of making lamellar sheets of fluorphlogopite mica



United States Patent Ofiiice 3,100,18fi Patented Aug. 6, 1963 3,100,186 METHOD OF MAKiNG LAMELLAR SEEETS OF FLUORPHELOGOPITE MICA William McNeil! and Joseph E. Chrostowski, Philadelphia, and Thomas J. Maclms, Upper Darby, Pa., assignors to the United States of America as represented by the Secretary of the Army No Drawing. Filed June 24, 1969, Ser. No. 38,673 3 Claims. (Cl. 204181) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to improved dielectric materials and more particularly to an electrophoretic method of reconstituting synthetic mica into lamellar sheets for electronic applications.

Mica is one of the strategic materials of this country. They define a group of alumino-silicate materials possessing high dielectric strength, and have mechanical and thermal properties which make them especially suited for various electronic applications.

At the present time, a major proportion of high quality natural mica used in this country is obtained from foreign sources. A process for the production of synthetic micas has been developed by The Electrotechnical Laboratory, U.S. Bureau of Mines, Norris, Tennessee, but the yield of crystals of sufiiciently large dimensions for electronic applications has been wholly inadequate.

Consequently, attempts have been made to reconstitute synthetic mica into large sheet-like structures which would be adequate for such applications. These methods have generally been deficient however in producing ample crystals approaching 2 inches or greater, the minimum size required for many electronic applications. For example, one of the earliest and most extensively used approaches to mica reconstitution employed the well-known papermaking technique. There, the synthetic mica was broken down into flakes by grinding in an aqueous medium and then permitted to settle out of such medium onto a suitable surface or screen in the form of a reconstituted mica sheet or mat. The sheet or mat was then dried and treated to obtain specific desired properties. The orientation of particles in such mica papers was not sufiiciently lamellar to allow them to be recrystallized into transparent pure mica sheets.

It is therefore a principal object of this invention to provide a method for producing mica deposits which are sufliciently lamellar to be recrystallized to transparent, pure mica sheets.

A further object of the invention is to identify the critical conditions under which synthetic fluorphlogopite mica can be made into large lamellar sheets by electrophoretic depositions.

The exact nature of this invention as -well as other objects. and advantages thereof will be readily apparent from a consideration of the following description and claims.

Briefly, these objects are attained in accordance with the present invention by passing a suspension of synthetic fluorphlogopite mica particles over an electrode for electrophoretic deposition thereon.

More specifically, the practice of the process of the instant invention comprises the following general steps:

(1) Reduce or comminute the synthetic fluorphlogopite crystals to provide a considerable yield of particles within the range of approximately 5 to 25 microns. (Grinding) (2) Separate and retain the particles of this size by sedimentation or other suitable means. (Separation) (3) Suspend the retained particles in a suitable medium. (Suspension) (4) Electrophoretically deposit the suspended particles to form a rigid, wet mass of crystals on an electrode or cathode. (Electrophoretic deposition.)

(5) Dry and strip the deposit. (Drying and stripping.)

Grindin'g The reduction or comminution of synthetic fiuorphlogopite crystals was performed in a suspension medium to prevent any possibility of the freshly exposed mica surfaces from becoming contaminated through oxygen and/ or moisture adsorption. Therefore, it was. desirable to preliminarily heat the mica at approximately 500 C. for several hours to remove any traces of moisture which may have been previously adsorbed thereon. Pure, anhydrous redistilled n-amyl alcohol was used as the liquid medium, although 2 ethyl hexanol, n-amyl alcohol plus Aerosol OT, isobutyl alcohol, isopropyl alcohol plus Aerosol OT and anisole plus Terigitol TD were found to be satisfactory. Aerosol OT is a solid anionic material and is a di-octyl ester of sodium sulfo-succinate, a product of American Cyanami-de Chemical Corp., New York, New York, and is a well known stabilizer, wetting and dispersing agent. Tergitol TD is a tetradecyl alcohol condensed with ethylene oxide and is a product of Carbide & Carbon Chemicals Co., New York, New York. By pure n-amyl alcohol is meant n-amyl alcohol purified by drying over anhydrous sodium sulfate, decanted, distilled, and the fraction collected between- 136" and Actual grinding was performed on a Waring Blender operating with a high speed cutting blade. A mixture containing approximately 2 liters of the n-amyl alcohol and 800 grams of fluorphlogopite crystals was thus ground for a period of 3 to 4 hours, after which time, many particles within the range of 5 to 25 microns. in their largest dimension were made available.

Separation Separation of the fluorphlogopite particles was accomplished by sedimentation of the aforedescribed wet ground mixture. This mixture containing the 5-25 micron particles and other particlesof assorted sizes, was doubled approximately in volume by the incorporation therein of additional n-amyl alcohol, allowed to stand for a sufficient period until layers containing particles of various sizes appeared. The layer containing particles substantially all of which are of the. order of 5-25 microns. was then removed by siphoning, although other conventional. means could be used satisfactorily.

Suspension The removed portion was. further diluted with additional anhydrous n-amyl alcohol to form a suspension yielding a fluorphlogopite concentration within the range of about 10-400 grams per liter of n-amyl alcohol. These concentrations were found to be amenable for successful electrophoretic deposition of the particles. The addition of approximately 1 gram Aerosol OT per liter of n-amyl alcohol aided in decreasing the flocculating tendencies of the suspension.

Electrophoretic Deposition other metals such as stainless steel, niobium, titanium, and gold plated copper sheet may be used advantageously. Current densities ranging from about 0.01 to 3.0 milliamperes per square inch of cathode surface with cell voltages from about 10 to' 1000 volts may be used satisfactorily with deposition times ranging between 5 seconds to 1 hour. Ambient temperature, or approximately 25 C., was used. The suspension was agitated by means of a simple magnetic type stirrer and was maintained in viscous flow parallel to the electrode surfaces. Maximum agitation or flow rates which produced non-turbulence were used, since we have found that turbulent flow resulted in deposits of non-uniform thickness. 'Lower flow rates, while yielding satisfactory deposits, consumed additional time before achieving deposits of equal thickness.

Deposits obtained with suspensions having fluorphlogopite particles larger than approximately 25 microns yielded a mechanically weak, non-compact and non-uniform product. When the particle size was reduced below about 5 microns, good, fiat overleafing of particles was prevented, and hence, suitable lamellar sheets of the material were not successfully achieved.

Drying and Stripping After the desired deposit is formed on the cathode, the voltage thereto is switched off and the excess suspension allowed to drain. The wet deposit is then dried slowly to prevent any sudden or rapid volatilization which would tend to disrupt the fiuorphlogopite particle laminae.

In stripping the dried deposit from the cathode, it should be borne in mind that the reconstituted mica will be used in capacitor dielectrics, vacuum tube spacers, and the like, and therefore must be available in sheets which are clean and smooth on both sides and yet be of sufficient dimensions for electronic applications. T this end, the following described method was found to be admirably suited for stripping dried deposits from the cathode. Both the cathode and dried deposit were heated for approximately 10 minutes in a furnace containing an air atmosphere at temperatures ranging between290-500 C. The mica sheet was then readily separated from the cathode, but in several instances, a mica film remained on the cathode. Clean partition between the cathode and deposit may be aided by the application of a film of the order of 1 micron thickness of either a hard or soft parafiin type wax to the cathode surfaces and allowing the film to dry thereon before immersion into the suspension prior to deposition.

As an example of the product obtained from practicing the process of this invention, a smooth deposit approximately 0.030 inch thick and having lamellar characteristics was obtained under the following optmized values of process variables:

Suspension medium: pure, anhydrous n-amyl alcohol Suspension concentration: 200 grams fluorphlogopite'crys tals per liter of n-amyl alcohol Particle size range: Substantially all of the particles are between 5 and 25 microns in their largest dimension Deposition current density: 0.5 milliampere per square inch of cathode surface It is apparent from the foregoing description that a novel method has been provided for the reconstitution of smooth, lamellar fluorphlogopite through electrophoretic deposition of suspended synthetic mica.

We claim:

1. A process for reconstituting a crystalline, transparent, smooth lamellar fluorphlogopite mica from synthetic fluorphlogopite which comprises the steps of heating the synthetic fluorphlogopite to remove any moisture adsorbed thereon, comminuting the dried synthetic fiuorphlogopite with a high speed cutting blade in a liquid medium selected from the group consisting of pure anhydrous n-amyl alcohol, 2 ethyl hexanol, isobutyl alcohol, and isopropyl alcoho], to form a mixture yielding particles substantially all of which are of the order of about 5 to 25 microns in their largest dimension and exhibit smooth surfaces and edges, diluting the comminuted mixture to approximately double its original volume with an additional amount of said liquid medium, forming layers having particles of different size ranges with respect to their largest dimension by allowing the diluted mixture to stand, one of said layers containing said particles in the 5 to 25 micron range, separating said one layer and further diluting same with more of said liquid medium to form a suspension of approximately 10 to 400 grams of fiuorphlogopite particles per liter of said liquid medium, pouring said suspension into an electrophoretic deposition cell, immersing a cathode between the parallel surfaces of a U-shaped anode, said anode and cathode being vertically disposed and having opposed parallel flat surfaces, agitating the suspension to provide maximum non-turbulent flow parallel to said flat surfaces, applying a voltage between said cathode and said anode to yield a current density of 0.01 to 3 milliamperes per square inch of cathode surfaces for a period of about 5 seconds to 1 hour until a deposit of fluorphlogopite particles is formed on said flat surfaces of said cathode, slowly air drying said deposit and stripping the dried deposit from said cathode by the application of heat thereto.

2. A process according to claim 1 including the step of adding about 1 gram of an anionic surface active agent for each liter of said liquid medium to form said suspension.

3. A process according to claim 1 wherein said liquid medium consists of pure anhydrous n-amyl alcohol.

References Cited in the file of this patent UNITED STATES PATENTS 903,404 Reverdys Nov. 10, 1908 2,250,435 Norris July 22, 194'1 2,936,218 McNeill et a1. May 10, 1960 3,001,571 Hatch Sept. 26, 1961 

1. A PROCESS FOR RECONSTITUTING A CRYSTALLINE, TRANSPARENT, SMOOTH LAMELLAR FLUORPHLOGOPITE MICA FROM SYNTHETIC FLUOROPHLOGOPITE WHICH COMPRISES THE STEPS OF HEATING THE SYNTHETIC FLUORPHLOGOPITE TO REMOVE ANY MOISTURE ABSORBED THEREON COMMINUTING THE DRIED SYNTHETIC FLUOROPHLOGOPITE WITH A HIGH SPEED CUTTING BLADE IN A LIQUID MEDIUM SELELCTED FROM THE GROUP CONSISTING OF PURE ANHYDOROUS N-AMYL ALCOHOL, 2 ETHYL HEXANOL, ISOBUTYL ALCOHOL, AND ISOPROPYL ALCOHOL, TO FORM A MIXTURE YIELDING PARTICLES SUBSTANTIALLY ALL OF WHICH ARE OF THE ORDER OF ABOUT 5 TO 25 MICRONS IN THEIR LARGEST DIMENSION AND EXHIBIT SMOOTH SURFACES AND EDGES, DILUTING THE COMMINUTED MIXTURE TO APPROXIMATELY DOUBLE ITS ORIGINAL VOLUME WITH AN ADDITIONAL AMOUNT OF SAID LIQUID MEDIUM, FORMING LAYERS HAVING PARTICLES OF DIFFERENT SIZE RANGES WITH RESPECT TO THEIR LARGEST DIMENSION BY ALLOWING THE DILUTED MIXTURE TO STAND, ONE OF SAID LAYERS CONTAINING SAID PARTICLES IN THE 5 TO 25 MICRON RANGE, SEPARATING SAID ONE LAYER AND FURTHER DILUTING SAME WITH MORE OF SAID LIQUID MEDIUM TO FORM A SUSPENSION OF APPROXIMATELY 10 TO 400 GRAMS OF FLUOROPHLOGOPITE PARTICLES PER LITER OF SAID LIQUID MEDIUM, POURING SAID SUSPENSION INTO AN ELECTROPHORETIC DEPOSITION CELL, IMMERSING A CATHODE BETWEEN THE PARALLEL SURFACES OF A U-SHAPED ANODE, SAID ANODE AND CATHODE BEING VERTICALLY DISPOSED AND HAVING OPPOSED PARALLEL FLAT SURFACES, AGITATING THE SUSPENSION TO PROVIDE MAXIMUM NON-TRIBULENT FLOW PARALLEL TO SAID FLAT SURFACES, APPLYING A VOLTAGE BETWEEN SAID CATHODE AND SAID ANODE TO YIELD A CURRENT DENSITY OF 0.01 TO 3 MILLIAMPERES PER SQUARE INCH OF CATHODE SURFACES FOR A PERIOD OF ABOUT 5 SECONDS TO 1 HOUR UNTIL A DEPOSIT OF FLUORPHLOGOPITE PARTICLES IS FORMED ON SAID FLAT SURFACES OF SAID CATHODE, SLOWLY AIR DRYING SAID DEPOSIT AND STRIPPING THE DRIED DEPOSIT FROM SAID CATHODE BY THE APPLICATION OF HEAT THERETO. 